Electrical submersible pumps (ESP) are widely used to pump oil production wells. A typical ESP has a rotary pump driven by an electrical motor. A seal section is located between the pump and the motor to reduce the differential between the well fluid pressure on the exterior of the motor and the lubricant pressure within the motor. A drive shaft, normally in several sections, extends from the motor through the seal section and into the pump for rotating the pump. The pump may be a centrifugal pump having a large number of stages, each stage having an impeller and diffuser. The pump may be other types, such as a progressing cavity pump.
Many wells produce both gas and liquid, such as oil and water. Centrifugal pumps do not function well pumping gas. Some ESP installations have gas separators to remove gas from the well fluid prior to reaching the pump intake. The gas discharges into the well casing and flows up to the wellhead.
Another technique employs a shroud that surrounds the ESP and is supported by the tubing string. The shroud may have an open lower end that is placed below the lowest perforations or openings in the casing. The upper end of the shroud would be closed, requiring all of the well fluid to flow downward alongside the shroud to reach the open lower end. A closed upper end system is usually set below the perforations. As the well fluid flow turns down to flow toward the shroud inlet, some of the gas will separate. The shroud alternately may be inverted with a closed lower end and an open upper end. Typically, the open upper end is positioned above the casing perforations. This placement requires all of the well fluid to flow upward to the open upper end. As the well fluid turns to flow downward into the shroud to the pump intake, some of the gas separates.
The motor of an ESP in a shroud is typically below the pump. If within an inverted shroud, a recirculation tube may be attached to the pump and extend down below the motor to divert some of the well fluid being pumped below the motor. The diverted well fluid flows back alongside the motor to the pump intake, thereby cooling the motor.
While these types of shrouds work well, in some wells the perforations extend over a great distance. If so, it is difficult to position the shroud effectively above or below the perforations. In other wells, the casing perforations or openings may be in a horizontal section, making it difficult to install a shrouded ESP in the horizontal section. The horizontal section may have a smaller diameter casing or liner.